A fingerprints detection apparatus by a capacitance detection method as shown in FIG. 7 is known (see, for example, Gazette of U.S. Pat. No. 5,325,442). The fingerprints detection apparatus includes a sensor portion, what is called a fingerprints sensor chip 2 in which a plurality of detection electrodes 4 are arranged like an array on the surface of a semiconductor substrate 3 through an insulation film 5, and an insulating protection film (surface protection film) 6 is formed so as to cover those detection electrodes 4. With this fingerprints detection apparatus, when a finger 1 is put on the protection film 6 of the fingerprints sensor chip 2, a capacitance Cs [Cs1, Cs2] formed between the detection electrodes 4 and the surface of the finger 1 depending on unevenness of a fingerprints is detected, whereby a so-called fingerprints pattern is lifted.
The capacitance Cs formed between the detection electrodes 4 and the surface of finger 1 becomes larger in its value at a protuberance of the fingerprints because of a shorter distance between the protuberance and detection electrodes 4, whereas it becomes smaller in its value at a recess of the fingerprints because of a longer distance between the recess and detection electrodes 4. Therefore, by detecting the capacitance Cs, the fingerprints detection apparatus can lift the fingerprints pattern 7.
The surface of the fingerprints sensor chip 2 described above is exposed because of its structure in which the surface of the chip is touched by a finger. Accordingly, a ball-point pen, a coin and the like may bump against the chip surface. For this reason, the protection film 6 needs to have a sufficient mechanical strength. Besides, the protection film 6 is required to have a high dielectric constant and also have a stopping performance against mobile ions for preventing the invasion of those ions. An example of the mobile ions is Na ions contained in sweat on the surface of finger.
At present, the protection film of silicon nitride, polyimide and the like are generally used in the semiconductor process. However, when these films are used for the protection film 6 of the fingerprints sensor chip 2 described above, a problem is raised that a crack will occur by, for example, an act of pricking the surface of the film with a pen point. If a crack occurs in the protection film 6, the mobile ions can not be prevented from invading, so that characteristics of a semiconductor device connected to each detection electrode 4, namely, a switching device (e.g. MOS transistor) formed on the semiconductor substrate 3 will change.
It is due to an insufficient mechanical strength of the protection film 6 itself that a crack occurs in the protection film 6, but a main cause thereof is insufficient Vickers hardness of the detection electrodes 4.
At present, wiring materials of aluminum or copper are generally used in the semiconductor process, but they are small in the Vickers hardness as compared with the protection film of silicon nitride, etc. If aluminum or copper is used in the detection electrodes, when an external stress F is applied thereto as shown in FIG. 5, then the detection electrodes 4 will bend first. Depending on the bending amount of the detection electrodes 4, the protection film 6 also bends and cracks 8 will occur in the protection film 6, further in the insulation film 5 thereunder.
In other words, a tolerance to crack of the protection film 6 depends on the Vickers hardness of detection electrodes 4 rather than on the mechanical strength of protection film 6 itself, and thus the tolerance goes smaller than the ability of protection film 6. This raises another problem.
The same is applicable to the wiring 9 beneath the detection electrodes 4. As is shown in FIG. 6, even if materials of high Vickers hardness is employed in the detection electrodes 4, when the Vickers hardness of wiring 9 is small, the wiring 9 will bend first if an external force F is applied. Depending on the bending amount of wiring 9, the detection electrodes 4 and also the protection film 6 will bend, thereby causing cracks 8 to be occurred in the protection film 6 and insulation film 5.